This invention relates generally to tuners used in television receivers or the like, and more particularly, the present invention relates to such a tuner of double superheterodyne system used for VHF and/or UHF band.
In conventional VHF television (TV) receivers, so called electronic tuners of single superheterodyne system are widely used. However, such a conventional tuner is capable of receiving only a relatively narrow frequency range, and therefore, it is necessary to employ two different tuners for converting both VHF and UHF bands. Furthermore, switching diodes are usualy used to change the receiving frequency range in VHF band so as to effect switching between VHF low band and high band. Moreover, such a conventional tuner suffers from unstable output characteristics, and therefore, it cannot be used as a tuner of CATV because it is necessary to process the receiving signal.
In order to remove the above-described drawbacks in conventional tuners, an improved all channel tuner of double superheterodyne system was recently divised. In such a known double superheterodyne system, the first intermediate frequency is usually to 600 MHz or so for receiving a CATV band ranging from 50 to 450 MHz. While this improved tuner, although the above-mentioned drawbacks have been removed, there arises a problem that spurious signals are apt to appear in the first intermediate frequency band, resulting in beat interference which appears as noises in a reproduced picture on a TV CRT. According to another known double superheterodyne system disclosed in Japanese Patent Provisional Publication No. 56-144642 or in a corresponding U.S. Pat. No. 4,340,975, the first intermediate frequency is set to a value higher than 2500 MHz so that undesirable spurious signals are suppressed. However, due to such an extremely high first intermediate frequency circuit elements are expensive, while there are various problems in manufacturing techniques, and therefore such a double superheterodyne tuner is not practical.